There are several families of semiconductor logic circuitry, each of which is defined at least partly by different voltage levels that they accept as input and provide as output. In addition, logic families may also specify how fast the logic needs to operate. In general, for semiconductor memories, such as Dynamic Random Access Memories, DRAMs, operating voltages are getting lower. Several factors are contributing to this lowering of voltage levels. Line widths are decreasing in size at the same time that the speeds of DRAM devices and other semiconductor devices are increasing. It is difficult to sink current into devices, with such small geometries, in order to change the voltage level. A change in voltage from high to low corresponds to a change in logic levels from or to a one or a zero. As the frequency of operation of such devices increases, such changes occur faster.
In order to change faster, the difference between the high and low voltages tends to decrease with each new logic family introduced. One such family is called Transistor-transistor Logic (TTL). The next iteration of logic produced in the TTL family is called Low Voltage TTL (LVTTL). Yet a further iteration of logic has produced the family known as Gunning Transceiver Logic (GTL). In the GTL family, more than just the voltage level has changed in order to increase the speed of operation. The output circuit for devices formed with such logic has also changed in order to obtain the increase in speed.
DRAMs for computers and other electronic devices are manufactured in very high volumes. Electronic device manufacturers design their devices to interface with such memories at various logic levels. Currently, memories are modified during their manufacturing process to provide custom input and output levels to accommodate each device's logic level requirement. This can result in differences early in the manufacturing process for memories intended for different devices. There is a need for reducing differences in the manufacturing process for each memory device with a different logic level interface. There is a further need to apply the process modifications toward the end of the manufacturing process of memory devices to better correlate manufacturing output to demand, which may change quickly. There is a further need to minimize the amount of space on a chip to accommodate input and output circuitry.
Some of the circuitry currently formed on a DRAM at the same time as the memory is being formed is shown in prior art FIGS. 1 and 2. In FIG. 1, LVTTL logic and TTL logic both use a pair of transistors or sets of transistors in parallel--a pull-up transistor 110 to pull an output line 114 up to the supply voltage 116 and a pull-down transistor 112 to pull an output line 114 down to ground depending on the logic level desired. These are very big transistors or sets of parallel transistors 110 and 112, and require high current to switch faster. In fact, at switching frequencies above 100 MHZ, they can not switch fast enough. The pull-up transistor 110 and the pull-down transistor 112 are coupled respectively to a pull-up control 118 and pull-down control 120, which are connected in parallel with integrated circuitry on a chip, such as a memory array 122 shown in FIG. 1.
In comparison, as shown in FIG. 2, the output circuit for GTL-terminated logic uses a single transistor 224 to drive an output line 226. The transistor 224 is a pull-down transistor 224, driven by a pull-down control 228. The GTL logic circuit is coupled to integrated circuitry on a chip, such as a memory array 222 shown in FIG. 2. The GTL logic circuit requires only about one-half the current of a LVTTL or TTL output driver and hence can switch much faster.
Small Swing Transistor Logic (SSTL) logic provides for fast switching by reducing the difference between high and low output voltage levels by reducing the width of the pull-up and pull-down transistors. By decreasing the overall swing in output voltage, faster switching times are obtained.
There is a need to provide the various output levels for semiconductor devices in a manner that makes them easy to manufacture. There is a need to provide modifications of such devices late in the process of making them to ensure that the devices manufactured meet the demand for the various output levels.